1. Field of the invention
This invention relates to a method of fabricating a semiconductor device, especially a method of processing an waste water generated in a step of forming a semiconductor device and reusing thereof
2. Description of the related Art
Generally, when grinding and polishing or milling of inorganic or organic solids such as metals and ceramics is carried out, fine particles are generated. These fine particles are generally washed away by means of a fluid such as water and discharged as wastewater or sewage. This invention relates to a system for reusing industrial wastewater.
Reducing industrial waste is a serious current ecological theme now and is an important business issue for the 21st century. Among industrial waste, there are various kinds of wastewater and sewage.
In the following description, water or other fluid substances containing matters to be removed will be called wastewater. Such wastewater has removable matters extracted by an expensive filtration processing apparatus or the like; the wastewater filtered becomes clean and is reused. Wastewater containing unremovable matters is further processed or disposed as industrial waste. Filtered water may be reused or returned to nature.
However, because of high plant costs related to running a filtration process, employing these apparatuses poses a very difficult problem.
Also, high costs of sewage treatment are a serious problem and therefore, a system with low initial cost and low running cost is urgently needed.
As an example, wastewater treatment in the semiconductor field will be described below. Generally, when grinding or polishing a metal, semiconductor or ceramic sheet, such factors as limiting temperature increases of equipment, improving lubrication, and preventing adhesion of waste produced by grinding and cutting the sheet are considered, and a fluid such as water is supplied to the sheet.
When semiconductor wafers are formed of ingot by grinding and slicing the ingot into wafers, or when a semiconductor wafer, which is a sheet of a semiconductor material, is diced or back-grinded, pure water is supplied. For preventing a temperature rise of a dicing blade in a dicing apparatus and for preventing dicing waste from adhering to the wafer, pure water is made to flow on the semiconductor wafer, or a nozzle for discharging water is mounted so that pure water will strike the blade and the wafer. Also, when the thickness of a wafer is reduced by back-grinding, pure water is supplied for similar reasons.
Wastewater containing semiconductor wafer grinding waste or polishing waste is filtered and thereby turned into clean water and returned to nature or reused, and concentrated wastewater is collected.
At present, there are two semiconductor manufacturing methods for processing wastewater containing mainly Si waste: a coagulating sedimentation method, and a method combining filtration and a centrifugal separator.
In the coagulating sedimentation method, PAC (poly-aluminum chloride) or Al.sub.2 (SO.sub.4).sub.3 (band sulfate), for example, is mixed with wastewater as a coagulant. Si reactants are produced, and the wastewater is cleaned by removing those reactants.
In the method of combining filtration and centrifugal separation, wastewater is filtered, and the concentrated wastewater is put in a centrifugal separator and collected as sludge. Water obtained by filtering the wastewater is discharged into nature or reused.
As shown in FIG. 13, wastewater produced during dicing is collected in a raw water tank 201 and fed by a pump 202 to a filtering apparatus 203. Because the filtering apparatus 203 is fitted with ceramic or organic filters F, filtered water is fed through a pipe 204 to a water tank 205 and reused, or it is discharged to the environment.
Since the filters F gets clogged, the filtering apparatus 203 is periodically washed. This is accomplished by, for example, closing a valve B1 on the raw water tank 201, opening a valve B3 and a valve B2 for feeding washing water to the raw water tank 201, and back-washing the filters F with water from the water tank 205. Wastewater with highly concentrated Si waste is returned to the raw water tank 201. Concentrated water in a concentrated water tank 206 is transported through a pump 208 to a centrifugal separator 209, and is separated by the centrifugal separator 209 into sludge and liquid. The sludge containing Si is collected in a sludge collecting tank 210, and the liquid is collected in a liquid tank 211. Also, water of the liquid tank 211 in which separated liquid is collected is transported through a pump 212 to the raw water tank 201.
Similar methods to those above have also been employed in the collection of waste produced by grinding and polishing solids and sheets having metal materials such as Cu, Fe, Al as their main waste material and solids and sheets made of inorganic materials such as ceramics.
In the coagulating sedimentation method, a chemical as a coagulant is mixed with the filtered water. However it is very difficult to determine the necessary and sufficient amount of chemical that the Si waste will completely react with, and inevitably excess chemical is introduced and some chemical will remain unreacted. In contrast, if the amount of chemical is low, not all the Si waste will coagulate and some Si waste will remain in the solution.
When the amount of chemical is excessive, some chemical remain in the supernatant liquid. The supernatant liquid may not be reusable because of possible undesirable chemical reactions with the excess chemical. For example, filtered water with the excess chemical cannot be reused on a wafer during dicing, because it causes an undesirable chemical reaction.
Floc, a reactant of chemical and silicon waste, is produced as a suspended solid. For forming floc, the pH conditions are strict; pH of about 6 to 8 must be maintained using an agitator, a pH measuring apparatus, a coagulant pouring device and control devices. For example, for a wastewater processing capacity of 3 m.sup.3 /hour, a tank with a diameter of 3 meters and a depth of 4 meters (a sedimentation tank of about 15 m.sup.3) would be necessary, and it becomes a large system requiring an installation area of about 11 meters.times.11.
Furthermore, some floc may not settle and drift out of the tank, making the collection difficult. Hence, there are problems such as a high initial cost of the filtration system because of the plant size, difficulties of reusing the filtered water, and a high running cost of the system because a chemical is used.
The reuse of water is possible with the method combining filtration and a centrifugal separator of 5 m.sup.3 /hour because filters F (those made from polysulfone fiber, called UF modules, or ceramic filters) are used in the filtering apparatus 203. However, four filters F are installed in the filtering apparatus 203, and because the life of the filters F is about a year, it is necessary to replace the expensive (approximately 500,000 yen/unit) filter at least once a year. Furthermore, the load on the motor of the pump 202 for applying water to the filtering apparatus 203 is large because the filtration method is such that the filters F are of a pressurized type, and the pump 202 is a high-capacity type. Of the wastewater passing through the filters F, about 2/3 is returned to the raw water tank 201. Also, because wastewater containing silicon waste is transported by the pump 202, the internal walls of the pump 202 are scratched, cutting the life of the pump 202 short.
Hence, the cost of electricity for the motor is high and the running cost is also very high because there are replacement costs of the pump P and the filters F. FIG. 12 shows comparative data of the above system and a system of the invention described in the disclosure below. There are problems such as the size of the system, replacement of the filters, washing of the filters, and running costs.
To remove solid matter damaging to the earth's environment as much as possible, various devices must be added, and therefore, the filtration system, must necessarily becomes large, leading to enormous initial costs and running costs.